The Pale Green Dot: A Method to Characterize Proxima Centauri b Using Exo-Aurorae

Abstract

We examine the feasibility of detecting auroral emission from the potentially habitable exoplanet Proxima Centauri b. Detection of aurorae would yield an independent confirmation of the planet's existence, constrain the presence and composition of its atmosphere, and determine the planet's eccentricity and inclination, thereby breaking the mass-inclination degeneracy. If Proxima Centauri b is a terrestrial world with an Earth-like atmosphere and magnetic field, we estimate the power at the 5577Å OI auroral line is on the order of 0.1 TW under steady-state stellar wind, or ∼100× stronger than that on Earth. This corresponds to a planet-star contrast ratio of 10 −6 − 10 −7 in a narrow band about the 5577Å line, although higher contrast (10 −4 − 10 −5 ) may be possible during periods of strong magnetospheric disturbance (auroral power 1 − 10 TW). We searched the Proxima Centauri b HARPS data for the 5577Å line and for other prominent oxygen and nitrogen lines, but find no signal, indicating that the OI auroral line contrast must be lower than 2 × 10 −2 (with power 3,000 TW), consistent with our predictions. We find that observations of 0.1 TW auroral emission lines are likely infeasible with current and planned telescopes. However, future observations with a space-based coronagraphic telescope or a ground-based extremely large telescope (ELT) with a coronagraph could push sensitivity down to terawatt oxygen aurorae (contrast 7 × 10 −6 ) with exposure times of ∼1 day. If a coronagraph design contrast of 10 −7 can be achieved with negligible instrumental noise, a future concept ELT could observe steady-state auroral emission in a few nights.

Detecting Ocean Glint on Exoplanets Using Multiphase Mapping

“…To simulate realistic observations of our VPL Earth Model spectra as an analog for a directly-imaged exoplanet, we use an open-source Python coronagraph noise model 3 , originally developed by Robinson et al (2016) for the WFIRST exoplanet direct imaging mission and expanded upon in subsequent studies (Luger et al 2017;Meadows et al 2018a). The model simulates planetary and background photon count rates as a function of wavelength using analytic parameterizations for telescope, instrumental, and astrophysical noise sources.…”

Future Exoplanet Research: Science Questions and How to Address Them

“…A similar phenomenon has been invoked earlier for the detection of exo-moons (Cabrera and Schneider 2007). Luger et al ( 2016) and Burkhart and Loeb ( 2017) have suggested the detection of exoaurorae on Proxima b. Similar aurorae should be detected on other nearby planets.…”

Surface and Temporal Biosignatures

“…However, it should be noted that, as Equation 4 shows, the fluorescence signature will be a separate surface source term rather than an enhancement in the reflectivity of the surface like a red-edge signal, which would mean the intensity of the signal would not be proportional to the surface irradiation by the star at that wavelength. Overlying gaseous absorption features and auroral activity (Luger et al 2017) will also complicate the interpretation of fluorescence biosignatures.…”